The present invention relates to an adsorbent useful for the cleaning of gas containing ketonic organic solvents and an use method thereof.
So far, for the cleaning of gas containing solvents, activated carbon has been used extensively as an adsorbent. When ketonic organic solvents are contained in gas, however, the formation of very small amount of decomposition products is often identified due to the catalytic action of activated carbon. Among these decomposition products, easily desorptive ones can cause a decrease in purity of recovered solvent and hardly desorptive ones can contaminate the activated carbon, both resulting in the obstacle for adsorption. Moreover, the decomposition products are most often acids, which adversely affect on the corrosion of the material of device.
Ketones produce carboxylic acids via enol intermediates due to the oxdative action on the surface of adsorbent. Because of the generation of heat, this reaction progresses abruptly and the heat of reaction generated is accumulated. When sufficient oxygen is supplied, chain reaction is triggered and the bed of activated carbon itself sometimes takes fire. The firing temperature of fresh activated carbon is around 400.degree. to 500.degree. C., but, if large amounts of high-boiling point substances are accumulated, it may sometimes decrease to lower than 200.degree. C.
As described, the use of activated carbon for the cleaning of gas containing ketonic organic solvents includes various problems. For this reason, contrivances have been made in such ways that an activated carbon with low catalytic activity is used, that a damping device is installed in the upstream area of adsorption bed to prevent the temperature rise of bed due to the heat of adsorption, heat of reaction, etc., and the like. However, as long as the activated carbon is used as an adsorbent, more or less catalytic action to ketonic organic solvents is inevitable. When installing the damping device, the inhibition of catalytic action will become possible, but, if the relative humidity becomes high, the adsorption level of activated carbon to organic solvents will be decreased. Thus, the greatest care was required for the running and the management of adsorption device.
Moreover, recently, zeolites with increased hydrophobicity also begin to be used for the adsorption of organic compounds as new adsorbents in place of activated carbon (WO 84/04913 and U.S. Pat. No. 4795482).
However, when the inventors tried the adsorption of ketonic organic solvents with these hydrophobic zeolites, it was found that they exhibited the catalytic activity similarly to activated carbon. Namely, while it is possible to adsorb the ketonic organic solvents by treating waste gas containing ketonic organic solvents with activated carbon, conventional hydrophobic zeolites or the like, the ketonic organic solvents cause the decomposition or the polymerization reaction on adsorbent in the process of the regeneration of adsorbent under heat due to the catalytic action thereof. As a result, the purity of ketonic organic solvents in recovered organic solvent was low, not permitting the reuse thereof. Moreover, during repeating the adsorption and desorption procedures, the adsorption performance of adsorbent itself was also decreased and not only stabilized cleaning of waste gas was impossible, but also problems from the points of safety and maintenance such as firing and corrosion of device were present.
Paying an attention particularly to the catalytic action of adsorbent to ketonic organic solvents, the inventors made diligent investigations to provide an adsorbent which enables the recovery of high-purity ketonic organic solvents from waste gas without special procedures and further which is not in dangers of firing and decreased adsorption performance and does not exhibit the catalytic property to ketonic organic solvents.